In an engineering first, Stanford University scientists have invented
a low-cost water splitter that uses a single catalyst to produce both
hydrogen and oxygen gas 24 hours a day, seven days a week.

The researchers believe that the device, described in an open-access study published today (June 23) in Nature Communications, could provide a renewable source of clean-burning hydrogen fuel for transportation and industry.

“We have developed a low-voltage, single-catalyst water splitter that
continuously generates hydrogen and oxygen for more than 200 hours, an
exciting world-record performance,” said study co-author Yi Cui, an associate professor of materials science and engineering at Stanford and of photon science at the SLAC National Accelerator Laboratory.

The search for clean hydrogen

Hydrogen has long been promoted as an emissions-free alternative to
gasoline. But most commercial-grade hydrogen is made from natural gas — a
fossil fuel that contributes to global warming. So scientists have been
trying to develop a cheap and efficient way to extract pure hydrogen
from water.

A conventional water-splitting device consists of two electrodes
submerged in a water-based electrolyte. A low-voltage current applied to
the electrodes drives a catalytic reaction that separates molecules of H2O, releasing bubbles of hydrogen on one electrode and oxygen on the other.

In these devices, each electrode is embedded with a different
catalyst, typically platinum and iridium, two rare and costly metals.
But in 2014, Stanford chemist Hongjie Dai developed a water splitter made of inexpensive nickel and iron that runs on an ordinary 1.5-volt battery.

In the new study, Cui and his colleagues advanced that technology further...

No information about the conversion efficiency. Is this overunity, for instance do we get more electric energy by converting the H2 O2 back into electric energy by means of a fuel cell????

It all depends on the current that is needed in order to split the water in this way, in other words it is irrelevant how high/low the input voltage is. Only the input/output power ratio is meaningful.

Splitting water is a two-step process, and in a new study, researchers have performed one of these steps (reduction) with 100% efficiency. The results shatter the previous record of 60% for hydrogen production with visible light, and emphasize that future research should focus on the other step (oxidation) in order to realize practical overall water splitting. The main application of splitting water into its components of oxygen and hydrogen is that the hydrogen can then be used to deliver energy to fuel cells for powering vehicles and electronic devices.

The 100% efficiency refers to the photon-to-hydrogen conversion efficiency, and it means that virtually all of the photons that reach the photocatalyst generate an electron, and every two electrons produce one H2 molecule. At 100% yield, the half-reaction produces about 100 H2 molecules per second (or one every 10 milliseconds) on each nanorod, and a typical sample contains about 600 trillion nanorods...